1. Field of the Invention
The present invention relates to an image pickup lens system and particularly, to an image pickup lens system which is comprised of two lenses and used in an image pick-up device, e.g., a CCD camera utilizing an image pickup element such as CCD, CMOS and the like mounted on a potable computer, a visual telephone and the like, and which can be reduced in size and weight.
2. Description of the Related Art
In recent years, multimedia have been developed remarkably, and for example, the demand for a camera utilizing an image pickup element such as CCD, CMOS and like, e.g., a CCD camera mounted on a portable computer, a visual telephone and the like is being increased remarkably. Such a CCD camera is required to be mounted in a limited space and for this reason, it is desired that the CCD camera is small in size and lightweight.
Therefore, it is also desired that a two-lens system using two lenses is conventionally used as such an image pickup lens system.
There are image-pickup lenses used in such a conventional two-lens system, which are disclosed, for example, in Japanese Patent Application Laid-open No.10-104511 and Japanese Patent Publication No.7-50246 and the like.
In the lens system using the image pickup lenses disclosed in the above Patents, the first and second lenses are disposed sequentially in the named order from the side of an object. The first lens is a meniscus concave lens having the same signs of radii of curvature at centers of first and second faces, and the second lens is a convex lens. Such a construction can ensure a back focal length, while shortening the focal length.
However, the conventional image pickup lens disclosed in Japanese Patent Application Laid-open No.10-104511 suffers from a problem that the radius of curvature of the center of the second face of the first lens is relatively small and for this reason, it is difficult to manufacture such lens. Another problem is that if a diaphragm is disposed at a location displaced from the second lens toward an image surface in order to correct the magnification chromatic aberration, the diameter of the first lens is increased and hence, it is further difficult to form the second face of the first lens. Such conventional image pickup lens suffers from a further problem that because it is difficult to form the second face, it is little possible to accommodate the angle-widening of the image pickup lens, i.e., the shortening of the focal length.
The image pickup lens disclosed in Japanese Patent Publication No.7-50246 has a fatal problem that the entire length of the optical system of the lens is increased by reason of the structure of the lens. The optical system of such an arrangement also suffers from the following problem: If stray light enters the lens system of the image pickup lens from outside an angle of view, a ghost is generated. Therefore, it is required that the shape of the first face of the first lens in the vicinity of an optical axis approximates to a planar shape. However, an increase in diameter of the first lens leads to a convex shape of the first face of the first lens, resulting in a reduction in performance of the image pickup lens. Moreover, the power of the first lens is largely associated with the generation of each aberration such as a distortion and the like and hence, there is also a problem that if the shape of the first face of the first lens is convex, the distortion and the like cannot be removed.